The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the speed of an engine in accordance with driving conditions of the vehicle.
A known control system for a continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of the engine is transmitted to driving wheels of the vehicle through a clutch and the transmission. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
When starting of the vehicle, the transmission ratio is set at a maximum value. When an accelerator pedal is depressed to increase engine speed, the speed of the vehicle increases, keeping the transmission ratio at the maximum value. A line 1.sub.1 of FIG. 5 shows the increases of engine and vehicle speeds. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to reduce (to upshift). At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a changing speed.
When the accelerator pedal is released, engine speed and vehicle speed reduce at a constant transmission ratio along a line, for example line 1.sub.2 of FIG. 5. At that time, although the throttle valve of the engine is at an idle position, the engine is driven by the driving wheels through the transmission and clutch at a speed higher than the idle speed of the engine. When the engine speed decreases to a set speed at a point A of FIG. 5, the transmission ratio begins to increase, while the engine speed is kept at the set value N. Thus, the vehicle speed reduces along a line 1.sub.3. The engine speed N is set to a relatively higher value in order to prevent stalling of the engine. Accordingly, the noise of the engine is loud compared with a motor vehicle having a manual transmission and fuel consumption increases.